Cyanate ester compounds produce triazine rings by curing and have high heat resistance and excellent electrical properties and therefore are widely used as raw materials of various functional polymer materials such as structural composite materials, adhesives, electrical insulating materials, and electrical and electronic components. However, in recent years, with an advance in required performance in these application fields, various properties required as functional polymer materials have become increasingly strict. Examples of such properties include flame retardancy, heat resistance, a low coefficiency of thermal expansion, low water-absorbing property, a low dielectric constant, a low dielectric loss tangent, weather resistance, chemical resistance, and high fracture toughness. However, functional polymer materials have not always satisfied these required properties so far.
For example, a problem in the semiconductor package material field is that with the thinning of base materials, warpage occurs due to the mismatch of the coefficiencies of thermal expansion between a semiconductor chip and a base material. As means for solving this, it is required to improve the low thermal expansion and high heat resistance of the functional polymer material itself used for the base material. In addition, from the viewpoint of consideration on human bodies and environment, the use of lead-free solder has been promoted for the soldering of a printed wiring board. Also, from the viewpoint of resistance to a reflow step at a high temperature attended with the lead-free soldering, it has been desired to improve low coefficiency of thermal expansion and high heat resistance for a functional polymer material.
From the viewpoint of improving the flame retardancy of a functional polymer material, the functional polymer material conventionally contain halogen atoms or phosphorus atoms in some cases. However, the halogen atoms are likely to generate halogen gas having a risk of causing environmental pollution during combustion and are also likely to reduce the insulating properties of a final product. The phosphorus atoms often decline required properties other than flame retardancy (heat resistance, moisture resistance, and low water-absorbing property or the like). Hence, it has also been desired to improve the flame retardancy of the functional polymer material without containing halogen atoms or phosphorus atoms.
Furthermore, when a laminate for printed wiring board applications or the like is produced, a prepreg is made by first dissolving an pre-hardened monomer which is a precursor of a functional polymer material in a solvent, such as methyl ethyl ketone, to prepare a varnish, then impregnating a glass cloth with the varnish, and drying it. Therefore, it has also been required to improve the solvent solubility of the monomer.
As an example in which a hardened product of a single cyanate ester compound having low thermal expansion and heat resistance is obtained, the use of a bifunctional cyanatophenyl-based cyanate ester compound in which the hydrogen of a methylene group bonding cyanatophenyl groups to each other is replaced by a particular alkyl group (1,1-bis(4-cyanatophenyl)isobutane), has been proposed (see Patent Document 1). As an example in which a hardened product of a single cyanate ester compound having low thermal expansion and flame retardancy is obtained, the use of a cyanate ester compound having an aralkyl structure has been proposed (for example, see Patent Document 2). Furthermore, as examples for providing a hardened product of a simple cyanate ester compound having flame retardancy and heat resistance, the uses of a cyanate ester compound containing an isocyanuric acid skeleton (see Patent Document 3), a cyanate ester compound containing a triazine skeleton (see Patent Document 4), and a bifunctional cyanatophenyl-based cyanate ester compound, in which the hydrogen in a methylene group bonding cyanatophenyl groups is replaced by a biphenyl group (see Patent Document 5), and the combination of a bisphenol A-based cyanate ester compound with a cyanate ester compound containing an imide skeleton (see Patent Document 6) have been proposed.
In recent years, high integration and/or miniaturization of semiconductors that are widely used for electronic devices, communication devices, and personal computers or the like have been increasingly accelerated. With such high integration and/or miniaturization, various properties required for laminates for semiconductor packaging, which are used for printed wiring boards, have become increasingly strict. Examples of the required properties include low water-absorbing property, heat resistance after moisture absorption, flame retardancy, a low dielectric constant, a low dielectric loss tangent, a low coefficiency of thermal expansion, heat resistance, and chemical resistance. However, these required properties have not been necessarily satisfied.
Conventionally, a cyanate ester compound has been known as a resin for printed wiring boards having excellent heat resistance and electrical properties. For example, a resin composition containing a bisphenol A-based cyanate ester compound and another thermosetting resin or the like has been widely used as a material for printed wiring boards or the like. The bisphenol A-based cyanate ester compound has excellent properties such as electrical properties, mechanical properties, and chemical resistance or the like. However, in some cases, this cyanate ester compound is insufficient in terms of low water-absorbing property, heat resistance after moisture absorption, and flame retardancy. Hence, for the purpose of further improving such properties, studies regarding various cyanate ester compounds having different structures have been conducted.
As a resin having a structure which is different from that of the bisphenol A-based cyanate ester compound, a novolac-based cyanate ester compound has been frequently used (see Patent Document 7). However, the novolac-based cyanate ester compound is problematic in that it easily causes insufficient hardness, and in that the obtained hardened product has a high water absorption rate and decreased heat resistance after moisture absorption. Hence, prepolymerization of a novolac-based cyanate ester compound and a bisphenol A-based cyanate ester compound has been proposed as a method for solving these problems (see Patent Document 8).
As a method of improving flame retardancy, it has been proposed to use a fluorinated cyanate ester compound, or to mix a cyanate ester compound with a halogen-based compound or prepolymerize these compounds so as to allow a resin composition to contain the halogen-based compound (see Patent Documents 9 and 10).